Difference Between Roasting and Calcination

Main Difference – Roasting vs Calcination

Although the final outcome of roasting and calcination is the same, they are two different processes used to obtain metals from metal ores. The final outcome of the two processes is the conversion of an ore into an oxide ore. Roasting is the process of heating the ore in the presence of air. It involves gas-solid reactions that take place at very high temperatures. Calcination is heating to high temperatures in air or oxygen. However, the amount of air or oxygen here is limited. Therefore, the main difference between roasting and calcination is that roasting involves the heating of ore in the presence of excess air or oxygen whereas calcination involves the heating of ore in the presence of limited air or oxygen.

What is Roasting

Roasting is the heating of a metal ore in the presence of excess air or oxygen. This process involves solid-gas reactions. The aim of this process is to purify metals from their ore. An ore is a type of rock that contains sufficient minerals with important elements including metals. In roasting, the ore is heated below its melting point.

Figure 1: A Portion of a Roasted Ore

Oxidizing Roasting

Oxidizing roasting is the most common form of roasting. Here, the metal ore is heated in the presence of excess oxygen. The roasting causes the burning of impurities, partially or completely by oxygen. Roasting is done by treating the ore with very hot air. This technique is mostly used for sulphide ores.

The reactions that take place during the roasting process may include:

Oxidation

Oxidation occurs in the oxidizing roasting process. Here, the impurities bound with the metal are replaced by oxygen. Ex: Sulphur is replaced by oxygen.

Reduction

Reduction reactions may partially reduce an oxide ore before smelting process (smelting process is the process from by a metal extracted from its ore by heating and melting).

Sulphation

Sulphation reactions cause the formation of metal sulfides due to the reaction between sulfides in the feed (ore) and the metal. These reactions occur under controlled temperatures and gas conditions.

But if the temperature and gas conditions are maintained, we can obtain a metal oxide and a metal sulfide from a mixed sulfide feed (a mixture of sulfides of different metals. Ex: iron sulfide and cooper sulfide mixture). This process is known as “selective sulphation”.

Pyro-hydrolysis

Pyrohydrolysis is a chemical process that take place in the presence of water vapor (at about 300oC). It is useful in forming the metal oxide from a metal chloride. (But all metal chlorides cannot undergo pyrohydrolysis if they do not have a negative Gibbs energy for decomposition and a low volatility).

Ex: MxCly + nH2O → MxOz + yHCl

M is metal, n is the number of water molecules that react, y is the number of HCl molecules formed and x and z are number of metal and oxygen atoms present in each molecule respectively.

The roasting of sulphide ores converts the sulphide into oxide and sulphur into sulphur dioxide. However, this process releases a high amount of toxic gases and toxic substances such as arsenic.

What is Calcination

Calcination is the heating of a metal ore in the presence of limited air or oxygen. In calcination, the ore is heated to a temperature below its melting point. This process is done mainly to remove volatile impurities. The name calcination is derived from a Latin name due to its major application; the heating of calcium carbonate ores.

In calcination, the ore is heated to a temperature below its melting point. Calcination is done in a reactor, which is a cylindrical structure called calciner. In this reactor, the calcination is done under controlled conditions. Here, carbon dioxide is produced and released during the calcination and the calcium carbonate is converted into calcium oxide. This process is done mainly to remove volatile impurities.

Figure 2: Furnace used for Calcination

But sometimes a furnace is used for the calcination because heating a substance into very high temperatures is done inside a furnace.

A typical example for calcination is the production of lime from limestone. There, limestone is given a high temperature that is enough to form and release carbon dioxide gas. Lime is produced in an easily powdered condition.

Difference Between Roasting and Calcination

Definition

Roasting:Roasting is the heating of a metal ore in the presence of excess air or oxygen.

Calcination:Calcination is the heating of a metal ore in the presence of limited air or oxygen.

Oxygen or Air

Roasting:Roasting requires an excess amount of air or oxygen.

Calcination:Calcination is done at limited air or oxygen.

Major Application

Roasting:Roasting is mainly done for sulphide ores.

Calcination: Calcination is done for carbonate ores.

Toxicity

Roasting:Roasting releases toxic gases and substances.

Calcination: Calcination releases toxic, volatile compounds.

Conclusion

Roasting and calcination are thermal processes. The main difference between roasting and calcination is that roasting involves the heating of ore in the presence of excess air or oxygen whereas calcination involves the heating of ore in the presence of limited air or oxygen.

About the Author: Madhusha

Madhusha is a BSc (Hons) graduate in the field of Biological Sciences and is currently pursuing for her Masters in Industrial and Environmental Chemistry. Her interest areas for writing and research include Biochemistry and Environmental Chemistry.